Multimode optical fibers are used for short-distance applications and local networks. The core of a multimode fiber generally has a diameter of approximately 50 microns (μm) and a numerical aperture greater than 0.2. By way of comparison, a single-mode fiber generally has a diameter of approximately 8 to 9 microns (μm) and a numerical aperture greater than 0.12. Thus, for a particular wavelength, several optical modes propagate simultaneously along the fiber, carrying the same information. The bandwidth is directly linked to the group velocity of the optical modes propagating in the multimode core of the fiber. To guarantee a high bandwidth, it is necessary for the group velocities of all the modes to be identical. In other words, the intermodal dispersion (i.e., the difference in group velocity between the slower mode and the faster mode) should be minimized for a particular wavelength. The multimode fibers have been the subject of international standardization under standard ITU-T G.651, which, in particular, defines criteria (e.g., bandwidth, numerical aperture, and core diameter) that relate to the requirements for optical fiber compatibility. The standard ITU-T G.651 is hereby incorporated by reference in its entirety.
To reduce the intermodal dispersion in a multimode fiber, it has been proposed since the 1970s to produce graded-index fibers with a parabolic core profile. Such an optical fiber has been used for many years and its characteristics have been described in particular in the publications “Multimode Theory of Graded-Core Fibers” by D. Gloge et al., Bell System Technical Journal 1973, pp. 1563-1578, and “Pulse Broadening in Graded-Index Optical Fibers” by Olshansky et al., Applied Optics, Vol. 15, No. 2, February 1976. Each of these publications is hereby incorporated by reference in its entirety.
A graded-index profile can be defined by a relationship between the index value n at a point as a function of the distance r from this point to the center of the fiber:
  n  =            n      1        ⁢                  1        -                  2          ⁢                                    Δ              ⁡                              (                                  r                  a                                )                                      α                                              wherein,        α≧1; (α→∞ corresponding to a step-index profile);        n1 is the maximum index of the multimode core;        a is the radius of the multimode core; and        
      Δ    =                  (                              n            1            2                    -                      n            0            2                          )                    2        ⁢                  n          1          2                      ;                wherein,        n0 is the minimum index of the multimode core, generally corresponding to the index of the cladding (typically made of silica).        
A multimode fiber with a graded index therefore has a core profile with a rotational symmetry such that along any radial direction the value of the index decreases continuously from the center of the fiber to its periphery. These curves are generally representative of the theoretical or target profile of the optical fiber, though fiber-manufacturing constraints may lead to a slightly different profile.
When a light signal propagates in such a core having a graded-index, the different modes experience a different propagation medium, which affects their speed of propagation differently. By adjusting the value of the parameter α, it is therefore possible to theoretically obtain a group velocity that is virtually equal for all the modes and thus a reduced intermodal dispersion for a particular wavelength. A value for the parameter α of between 1.8 and 2.2 generally allows a satisfactory limitation of the modal dispersion.
That said, an optimum value of the parameter α is valid for one particular wavelength. Thus, a multimode fiber typically transmits a monochromatic optical signal having a particular wavelength for which the alpha (α) profile of the fiber has been optimized. U.S. Pat. No. 6,363,195, which is hereby incorporated by reference in its entirety, proposes to compensate for the modal dispersion of a multimode optical link by using a concatenation of multimode fibers in order to optimize the bandwidth for two transmission windows, one centered on 850 nanometers and the other on 1300 nanometers. This patent proposes to use a length of a first multimode fiber having a value of parameter α1 of between 0.8 and 2.1 to optimize the bandwidth at 850 nanometers and a length of a second multimode fiber having a value of parameter α2 between the first value α1 and 8 to optimize the bandwidth at 1300 nanometers.
Furthermore, the exact parameter value α is difficult to control during fiber manufacturing. In order to compensate the profile deviations from a theoretical profile having an optimum value α, U.S. Pat. No. 7,139,457 proposes a concatenation of multimode fibers. The alpha (α) profile of each fiber and the length of each fiber are optimized in order to maximize the bandwidth over a given optical link. U.S. Pat. No. 7,139,457 is hereby incorporated by reference in its entirety.
Using fiber concatenations for modal dispersion compensation leads to more complex and more costly optical systems. Moreover, the foregoing documents are not concerned with the spectral dispersion of the sources used.
In this regard, the sources used in optical transmission systems are not generally monochromatic. By way of example, the widely used vertical-cavity, surface-emitting diode lasers (VCSEL) have a wide-spectrum discrete emission. The VCSELs used in high-speed transmissions are generally longitudinally, but not transversally, single mode. Each transverse mode of the laser has its own wavelength corresponding to the various peaks of the emission spectrum (i.e., the emission spectrum has a spatial dependence).
Accordingly, a multimode fiber has an alpha profile with a value of parameter α optimized for one particular wavelength. Thus, the introduction of a polychromatic optical signal originating from a transverse multimode source in a multimode fiber causes the appearance of a modal dispersion and consequently a reduction in the bandwidth.
U.S. Patent Publication No. 2004/0184492, which is hereby incorporated by reference in its entirety, proposes to use only one single transverse mode of a VCSEL source by conditioning the emitted signal before its introduction into the multimode fiber. The use of a single transverse mode of a VCSEL source however greatly reduces the power of the emitted signal and leads to a reduction in the power received by an optical receiver at the end of the line, resulting in a reduction in the performance of the optical system. Moreover, filtering part of the transverse modes of the VCSEL increases the relative intensity noise (RIN). In this regard, reference may be made to the publication of A. Gholami et al., “Optimization of VCSEL Spatiotemporal Operation in MMF Links for 10-Gigabit Ethernet” IEEE Journal of Selected Topics in Quantum Electronics, Vol. 12, No. 4, July/August 2006.
U.S. Patent Publication No. 2005/0078962, which is hereby incorporated by reference in its entirety, proposes to offset the VCSEL output from the center of the multimode fiber in order to introduce the signal into a “large bandwidth zone.” Such an offsetting, however, is difficult to control and needs to be adjusted for each fiber. Moreover, it may introduce substantial power loss, which is detrimental to the transmission quality.
British Patent No. 2,399,963, which is hereby incorporated by reference in its entirety, discloses a plurality of transverse modes of a polychromatic optical signal launched in a multimode fiber using a launch technique that restricts the number of modes launched into the fiber. In particular, this document suggests limiting the proportion of encircled flux launched into the fiber within a certain radius from the center, and limiting the radius within which a higher proportion of encircled flux is launched. A disadvantage of this kind of filtering is that it decreases the signal-to-noise ratio. Moreover, it does not resolve the issue with respect to wavelength dependency of modal dispersion of each propagation mode of the fiber.
A need therefore exists for an optical system using transverse multimode polychromatic sources, which has a broad effective bandwidth without excessive loss of the power emitted by the source.